1. Field of the Invention
This invention relates to ranging techniques and particularly to a method for continuously calibrating the velocity of an acoustic signal in a medium to better determine the range and position of an object relative to a first point.
2. Discussion of the Related Art
In order to determine the distance of an object in a medium using acoustic signals, it is important to know the propagation velocity of the acoustic signal in the medium. It is well known that the velocity of an acoustic signal is affected by the density and temperature of the medium. These factors are not easily determined in real time so another technique is often used which requires that the distance between two points be known. Given a known distance, velocity is readily calculated by measuring the travel time of the acoustic signal.
In aqueous environments the acoustic velocity may be roughly approximated by reference to a table. For example a table from a physics handbook notes that the velocity of sound in seawater of temperature 20 degree Celsius is approximately 1531 meters per second. This approximation is inappropriate in instances involving large distances because of density and temperature variations. In offshore environments acoustic velocities are traditionally determined between two ships. Both ships are usually equipped with radio navigation systems which provide the exact location of each ship and thus the distance therebetween. One ship may carry the acoustic source and the other ship (slave ship) may tow a detector such as a hydrophone. With the distance known and the travel time of the signal readily measurable, the velocity calculation is elementary.
The major disadvantage to this technique is the need for a slave vessel to determine the distance. This technique also requires that the slave ship contain expensive radio navigation equipment to determine its position relative to the other vessel. Such systems are disclosed in U.S. Pat. Nos. 4,669,067, 4,641,287, 4,532,617 and 4,513,401.
Another type of position determining system utilizes several submerged transponders secured to the ocean floor at surveyed locations. Each transponder generates a distinguishable acoustic pulse upon receipt of command signal from the vessel passing overhead. The acoustic signals from each transponder are received by hydrophones housed in the towed streamer cable and on the ship. The location of each hydrophone is triangulated from the signals sent from the transponders. These types of systems do not require the velocity of sound in the water since triangulation is used.
A major disadvantage in triangulation systems is the need to deploy the transponders at surveyed points on the ocean bottom. Typically the transponders are non-recoverable and costly. Such systems are disclosed in U.S. Pat. Nos. 4,635,236 and 4,555,779.
Another method of measuring distance is disclosed in U.S. Pat. No. 4,376,301. In this system, the velocity of sound in water is determined between the vessel and a first detector located in the end of the cable near the vessel. It is assumed that the distance between the source and the detector is a straight line and that the exact distance is known. That is the cable is paid out to the correct distance every time the cable is deployed. It is further assumed that the propagation velocity is constant along the length of the cable. Herein lies an assumption which may result in error. As the ship passes through the water, the propeller and the hull of the vessel introduce turbulence into the water which cause the water layers to swirl and mix, bringing cooler waters to the surface The cooler water may not mix effectively within the distance between the vessel and the near hydrophone, and in truth, the temperature of the water may vary considerably along the entire length of the cable. For every one degree change (Celsius) in water temperature, the velocity changes by approximately two meters per second. Depending upon the situation, the water temperature may vary to cause wide variations in the correct propagation velocity. Another disadvantage of such a system is that often, the source array used to generate the seismic signals is deployed directly behind the vessel and just ahead of the streamer cable. The acoustic source often consists of several air guns which when fired introduce large volumes of air into the water, forming bubbles of different sizes. The small bubbles are often still in the water when the streamer cable is towed past the firing point in the water. These small bubbles will cause variations is the acoustic velocity, thus the measurements from the ship to the cable may be distorted by the air.
It is an object of the present invention to provide a method for continuously calibrating the propagation velocity of an acoustic signal without the aid of a slave vessel.
It is another object of this invention to provide a method for calibrating the propagation velocity of an acoustic signal without knowing the distance between the transmitting and receiving points.
It is yet another object of the invention to provide a method for readily measuring or calculating the distance between a first and a second point without using fixed triangulation techniques.
It is still another object of this invention to provide a method for measuring distances between a first point and at least three other points located along a line laterally offset from the first point.